Such thermometer arrangements have been known for a long time and are described, for example, in EP 0 291 571 A1. Such a thermometer essentially comprises a measurement insert which is accommodated in a protective tube which is closed at one end and has a process flange, the closed end of the protective tube being immersed, as far as the process flange, in a container in which the medium whose temperature is to be determined is situated. The open end of the protective tube is guided into a connection housing in which at least connection terminals for connecting the measurement insert to a remote measured value processing system via a communication line are arranged. In addition, an electrical circuit for preprocessing measured values may be arranged in the connection housing.
The fundamental components of the thermometer arrangement are standardized in order to ensure that they can be replaced; the connection heads are thus standardized in DIN 43729 and the protective tubes are standardized in DIN 43772. In addition, thermometers having an exchangeable measurement insert are standardized in DIN 43770 and DIN 43771 and thermocouples, in particular, are standardized in DIN 43733.
The protective tube is composed of an abrasion-resistant and/or corrosion-resistant material and protects the measurement insert from attack by aggressive media. Protective tubes which are drilled from solid material and whose outer contours are machined are preferably used for high loads. This has the advantage that the mass, shape and wall thicknesses can be optimally adapted to the operating loads, such as pressure and flow, and to the metrological requirements, such as response time. However, protective tubes produced in this manner are complicated to produce and are accordingly expensive. For this reason, drilled protective tubes are used only for that area of the temperature sensor which comes into contact with the medium. Thermometers are extended with so-called neck tubes, if required, outside the area of the medium.
Depending on the field of application, the protective tubes are subject to particular legal requirements. In the form of pressure-bearing parts, for example, the material, design, calculation, production and testing must meet the requirements of the pressure vessel or steam boiler decree. The corresponding rules and regulations of country-specific codes, for example ASME codes, are also internationally widespread. In areas subject to explosion hazards, protective tubes are used to separate zones with different hazard frequencies.
Electrical devices inside areas subject to explosion hazards and associated operating means outside areas subject to explosion hazards must conform to EU Guideline 94/9/EC. An office nominated by the EU Member States checks compliance with the relevant standards and carries out requisite tests. Conformity is certified with an EC design test certificate or, for simple electrical operating means, including thermocouples, for example, according to point 5.4 of EN 50020, by means of a manufacturer's declaration of conformity. In addition, manufacturers must subject the production of these devices to a regular inspection audit by a nominated office.
The intended use of the devices and the associated operating means is a basic prerequisite for explosion protection. In addition to the special requirements for planning, selecting and erecting electrical installations in areas subject to explosion hazards, the manufacturer's information, including the EC design test certificate, labeling on the device and the associated operating instructions must be heeded in this respect.
The process of designing a temperature sensor for applications in the explosion-proof area has measures for avoiding explosions. These include, in particular, the intrinsically safe design of the circuits and/or pressure-resistant encapsulation of the measuring transducers in the connection head.
Since the known embodiments of industrial temperature sensors cannot rule out the entrance of an explosive gas mixture into the interior of the protective tube, the temperature range in which they are used must be limited to the temperature class of the gas mixture which is possibly in the surrounding area and thus also in the interior of the protective tube and/or in the process. Corresponding details on the labels make this clear by means of the maximum temperature of the medium Tmed, which depends on the temperature class T1 . . . T6. In this case, the temperature class indicates the maximum permissible surface temperature at which it can still be assumed, taking into account safety margins, that the gas will not ignite. If the temperature class of the medium and the temperature class of the surrounding area are identical, this situation constitutes virtually no restriction at all since it is not physically expedient to aim for measurement temperatures above the temperature class of the medium.
If, however, the temperature class of the medium were, in principle, to allow a higher measurement temperature than would result from the temperature class of the surrounding area, because, for example, a gas mixture with a higher ignition temperature or a gas which is not explosive at all is present in the process and/or because the interior of the protective tube is brought to a higher temperature as a result of intrinsic heating of the sensor, the theoretically (physically) possible measurement range would be restricted in an undesirable manner on account of the situation described above.
Despite a large amount of resistance, it is not possible to preclude damage to the protective tube. As a result of such damage, the medium advances through the interior of the protective tube into the connection housing and attacks the components of the electronic circuit and of the measurement insert and the insulation of the communication line.
The utility model DE 91 08 581 U1 discloses that an electrical measurement bushing, which is composed of a bushing body having an integrated collar made from fluorocarbon resin, is provided in order to seal the interior of the thermometer with respect to the outside world. The collar is arranged between flanges of the measuring tube and of the connection housing. In this case, the collar seals the interior of the measuring tube with respect to the connection housing and the outside world. However, the multipart construction is very complicated and makes it difficult to exchange the measurement insert.